As with most builds, the right race can be vital to your success. Here I will rate many races and templates with the classic
Navy-> Perfect,
Blue-> Great, Black-> Neutral,
Red-> Bad,
Maroon-> Never consider this.
Races
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Player's Handbook Races
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Humans: As almost always, a great choice for race. Since theiving is almost entirely skill-based, the extra skill points are a huge boon. The extra bonus feat is nice too as it can help you meet prerequisites for some nice Prestige Classes.
Dwarf: Dwarfs have only one real advantage in this archtype: Stonecunning. Even then, chances are most traps you're worried about aren't built into or out of stone. The save bonuses are nice, but if you're doing your job right, you aren't too worried about those. Darkvision is also a boon, but it's easy to get. The special bonuses for movement don't matter as you will never be wearing medium or heavy armor anyway.
Elf: normally a poor race choice, Elves have several things going for them as thieves. First off is the Dexterity bonus. This is kind of a big deal as Dexterity is the most important stat for thieves (followed by Intelligence). The free weapon prficiencies aren't anything to be ashamed of since most of the classes good for this archtype aren't proficient with the Longbow, but they're generally not that useful. The real kicker here is the often overlooked keen senses. You want those Spot/Listen/Search bonuses and the ability to auto-search secret doors can prove invaluable on the job.
Gnome: The spell-like abilities could come in handy for causing a distraction (Ghost Sound), lighting something when nobody is around (Dancing Lights), or making sure nobody sees your muddy bootprints (Prestidigitation). Other than that, there's Small size and a listen bonus, but there are much better options available.
Half-Elf: Nothing here that the Elf doesn't do better. Skip it.
Halfling: Bonus to Dexterity, small size, and bonuses to a bunch of important skills in addition to a bonus on all saving throws if something goes awry. The only downside is the low speed and no Darkvision. They also have racial substitution levels.
Half-Orc: A penalty to an important stat and a bonus to a dump stat. Darkvision just ain't enough to save these guys. Skip them.[/spoiler]
Monster Manual Races
[spoiler]This section will be a little different as I will only list the PHB subraces and others good for the deal. Races that get attention in other books will be covered there instead.
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Races books
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Destiny
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Illumians: The power sigils are great and it really does add a lot to a thief, but one thing gets in the way: the power sigils. Yes, I just listed the same ability as their greatest boon and their greatest weakness. Here's why: many people forget the power sigils GLOW and emit light. If you're breaking an entering, having the hands-free light source is useful, but it also makes you much easier to detect. You can supress them, but then you lose their benefits. They do have a unique method of intereacting with Symbols and Glyph traps, though. So long as you make good decisions with suppression and keeping them up, you should be fine.
Aasimar: On the strong side of a Black rating, this comes from the Darkvision, the Wisdom boost (remember, it's useful), and the bonuses on Listen and Spot, two of your more important skills. Outsider type is interesting too, but doesn't add much in this case.
Doppleganger: Master infiltrators, these guys have a lot of tools at their disposal for a different method of getting in and getting out without being detected. The high racial HD and LA is painful though as it severely limits your skill points.
Half Ogre: No, no, no, no, NO!!!. Large size plus penalties to your two most important abilities. You really are better off playing as a Half-Orc thief, and that's saying something.
Mongrelfolk: Not your best choice as they have a penalty to Intelligence. However, they do make up for it with numerous racial skill bonuses and the fact they can easily emulate other races for magic items. The Sound Imitation ability could be handy in some situations as well. This is a solid choice, but there are better options.
Sea Kin:If you're doing an aquatic game, these guys may work out decently. As is, it's not a good choice overall.
Sharakim: a penalty to Dex, but a bonus to Int. If it wasn't for the LA, these would be black, since they do have some bonuses for night time.
Skulk: Can you say yes please? A good bonus to Dex plus massive Hide and Move Silently bonuses (and the ability to move at full speed while taking no penalties on Hide), a constant Nondetection, and more difficult to be tracked means Skulks are very, very good thiefs. The only reason this isn't Navy is the fact it costs you 3 levels.
Tiefling: Bonus to Dex and Int, small racial bonus to Hide checks, Darkvision, and a spell-like ability to cover your escape if all goes to Hell. Navy if you're playing with LA buyoff.
Underfolk: Extra skill points, Darkvision, and racial bonuses on Hide and Listen make this a solid choice. Still on the weaker end of Blue, though, especially as True Seeing negates the Hide bonus (odd for racial).[/spoiler]
Dragon
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Dragonborn: Yeah, penalty to your most important stat and bonus to an unimportant one. The Wings and Senses aspects are useful, but there are much, much better options out there.
Kobolds: These guys have a lot going for them. Small size + Slight Build have them count as Tiny for squeezing and for Hide checks. They have a racial Dex bonus, and bonuses when dealing with traps in addition to Darkvision and a 30ft speed. The rest is just gravy, although their bonuses with Profession (Miner) means you could concievably manage to do a Bank Job-style break in. They also have racial substitution levels.
SpellscaleOnly low-light vision to contribute to a life of thieving and a bonus to a dump stat. Just no.[/spoiler]
Eberron
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Warforged: although they have no skill bonuses, no Dex or Int bonus, nor any special movement modes or senses, Warforged can make great thiefs. They have built-in armor with no armor check penalty nor maximum Dexterity bonus, a slew of immunities in case they screw up, and they can be incredibly paitient because they have no need to break to eat, sleep, or breathe. They also have access to Warforged components, some of which are quite good. Of note is the Warforged Scout which has all of this, a Dex bonus, and small size. This is one of the few situations where being a
Warforged Scout instead of a normal Warforged is a good idea.
Changeling: These guys aren't navy for the normal route of stealth and stealing. They're Navy because they can get in, get what they want, and get back out again without being detected while being seen by almost everyone. It's a sweet trick that only the Doppleganger can do better, but Changelings get away with 0 LA. Also, they have some nice substitution levels (if you use them, Rogue WILL be your first class level).
Shifter: The Dex bonus is nice, but the Intelligence takes a hit. Overall, this is probably a good trade. Some of the Shifter Traits (Cliffwalk, Swiftwing, Longstride) are amazing ways to get some extra movement when you need it. Shifters are on of two races capable of true flight at level 1, so this can be an extremely good move. It's worthwhile boosting your Con score for the longer shifting.
Kalashtar: Nothing much to say here. These guys get no penalties, nor any bonuses. The power points do, however, open up some psionic feats that might be worth your while. Mindlink can be useful to keep in contact with others during a multi-member raid.[/spoiler]
Faerun
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Artic Dwarves: Small size, but a Dex penalty and +2 LA. Other than that, same benefits and drawbacks as normal Dwarves. Immune to Cold, though.
Urdinnir Dwarf: Wow. Although the normal benefits make these guys no better than dwarves, the ability to pass right through stone and earth is amazing. That combined with the ability to shape metal and stone at will makes these guys incredible at getting into places where they shouldn't be. Falls flat when you're dealing with wood, but most of the time, they do exactly what you want. One of the few races worth the high LA for this task.
Wild Dwarf: Wild Dwarfs are small-sized and have automatic proficiency with a couple of useful weapons (handaxe especially). They lose Stonecunning, though, so it's a tradeoff. Poison Use might be handy, especially if you use that Blowgun proficiency to send out knockout poison.
Avariel: +4 Dex, +2 Int, +2 Wis, a fly speed, +4 bonus on Spot and Jump, plus the other elven skill bonuses and senses. Yeah, I would say this is probably worth the +3 LA, especially if you're playing with buyoff.
Any Orc variety: penalties to important attributes, and bonuses that don't matter. Skip them all.
Strongheart Halflings: Yes, yes, a thousand times yes. These guys take everything that's great about halflings and give you a free bonus feat to play with.
Fey'ri: Bonus to both Dex and Int, a fly speed, Darkvision, low-light vision, Alter Self at will (humanoid forms, not Outsider), and a list of abilities you get to choose from. Be careful though as some of the ablities boost the LA. As if this wasn't enough, you get a +2 bonus on numerous important skills. LA+2(or 3) keeps them from being navy.
Tanarrukk: Although a variety of Orc, this one does better in that it has no Int penalty, a Dex bonus, and it's an Outsider. The LA hurts, but at least the racial HD give you rogue skill points. Not a bad race for this job, but not great either.
Aarakocra: +4 Dex, an insane fly speed (90 ft!) and a +2 bonus on Listen & Spot. Why is this red? They're clausterphobic. The -4 penalty on attack rolls and skill checks whenever they're in an enclosed space is incredibly painful for a thief who spends the most crucial momens in such situations.
Dekanter goblins: +2 Dex, +2 Int, +2 Wis and Darkvision is NOT worth 6 class levels. Don't look again.
Kir-lanan: +2 Dex is good. Fly speed of 90 feet is good. +4 racial bonus on Hide is good. Spell-like abilities and Rebuke Undead are interesteing, but I'm not sure I'm willing to pay 8 class levels for it. It's better than a lot of ECL8 options, though.
Wemic: As centaur, but worse. Lower Dex bonus, lower speed, more HD and LA.[/spoiler]
Stone
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Goliath: These guys have Darkvision going for them as theifs and that's about it. They also have small bonuses with Climb and Jump. The Dex penalty makes me shy away from them.
Chaos Gnome: A little better than the Rock Gnome due to a Dex bonus and a luck re-roll, but this is canceled out by a +1 LA. Still not my first choice.
Dream Dwarf: A penalty to Dexterity, but the unique ability to see ehtereal creatures as easily as material ones. They also have longer-range Darkvision than normal dwarfs, but both of these benefits are situational and minor compared to the Dex penalty.
Feral Gargun: Appear better than the Goliath due to having a Dex bonus, but they are really, in all ways, inferior for thieving.
Stonechild: All right, even a thief would be impressed with those racial ability modifiers. This would be a pretty good choice if it wasn't for that +4 LA as you get Rogue skill points for the racial HD. Overall, not worth it.
Whisper Gnome: THE thief race. +2 Dex, small size, 30ft speed, Darkvision & low-light vision, +4 racial bonus on Hide and Move Silently (not counting the size bonus), +2 bonus on Listen and Spot, and some AMAZING spell-like abilities for the theif. Even Kender are hard-pressed to match these guys for stealing ability.[/spoiler]
Wild
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Raptoran: low-light vision is always useful, but the ability to jump out of a window for a quick escape is great. Later on, you get true flight. Also has a racial bonus on Spot and Climb checks and always knows which direction is North; a handy ability to keep you from getting turned around.
Catfolk: High speed and a good Dex bonus plus a racial bonus on Listen and Move Silently. Low-light vision too. This is an excellent race to use and gives a new meaning to the term "cat-burglar". I've seen this one in action and it's impressive.
Centaur: No! Large size = Bad! This is better than a lot of the Large races, though, because of the high speed and Dex bonus. This is partially cancelled out by an Int penalty, but it isn't nearly as much as most Large races.
Gnoll: They only have Darkvision going for them and an Int penalty, 2 racial HD, and +1LA against.
Killoren: These are decent for thieves, especially if you use the Aspect of the Hunter. A +2 racial bonus on 4 of the most important skills is nothing to sneer at.[/spoiler][/spoiler]
Other
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Still to do: MM[/spoiler]
Templates
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All templates here are only going to be those that would probably be chosen by a thief. Source will be listed.
Dark (Tome of Magic): Major, major awesome for a thief. +8 to Hide, +6 Move Silently, a speed boost for all movement forms, Superior low-light vision, Darkvision, and Hide in Plain Sight as an Extraordinary ability. Absolutely amazing for +1 LA, and you even get some Cold resistance for the heck of it.
Dragonborn (Races of the Dragon): See Races above.
Insectile (Savage Species): Pretty impressive. It gives a decent bonus to Dex and gives you a good spot bonus, but the extra arms and climb speed are the real goodies here. Tremorsense is great for detecting patrols before they detect you and Darkvision, of course, goes without saying. Worth the +2 LA
Lycant
rope (Monster Manual): This depends on your animal as to whether it's a good idea or not. You want low HD and high racial skill/Dex bonuses. Wererat works pretty well for this, but a lot of the felines are good too. Phinxkins from Dragon Magic are decent as they only have 1 HD and have a couple of nice features.
Shade (Races of Faerun): As a thief, darkness is your friend. As a Dark thief, darkness is your best friend. As a Shade thief, darkness might as well be the rest of the party. The template provides insane benefits, but only while you're in shadows or darkness and for a hefty +4 LA.
Shadow (Lords of Madness, Manual of the Planes): Movement boost (1.5 times prior speed for all movement forms), more abilities as you level up, and total concealment while not in full daylight. Awesome. Also gives you a +6 bonus to Move Silently and the night-vision benefits that Dark gives. Strangely, no Hide bonus, but you can make do. Worth the +2 LA.
Taur
ic (MM2, Savage Species): This one is really a grab bag. It depends what you grab for the lower half as to how good it is. I suggest forms with high Dex, other movement modes and/or high speed, and low HD. Leapord is a good choice as it keeps you medium, has good racial skill bonuses, and has a climb speed in addition to a decent land speed. Phynxkin (mentioned above) have fewer HD and the same speeds, but worse skill and ability bonuses.
Winged (Savage Species): possibly weaker than most of the other templates, this gives you flight with perfect manuverability (let's face it, you WILL have the 17 Dex for that). it also gives a good Dex boost and some extra Wisdom.
Incorporeal templates (varies): Being Incorporeal has many benefits and drawbacks for a thief. The benefits are that you cannot be detected via Touchsight, cannot be heard if you don't want to be, and can literally walk though solid objects. The downside is that you need to have the Ghostly Grasp feat if you expect to steal anything and you can't enter antimagic fields. The Phantom template from MMV is unique in that you are only incorporeal when you want to be. Very, very handy if you can get it past your DM.
Undead templates (varies): Being Undead has noteable benefits. For example, you can wait underwater indefinitely for the perfect opportunity to sneak in or away. You have no worry of poison, and undead/deathless/constructs with the Life Sense feat don't pick you up. Note that several templates for Undead and Incorporeal are the same.[/spoiler]